SRY Gene

Region that determines sex in therian mammals (including marsupials and placental mammals), the Y protein (SRY), also known as testis-determining factor (TDF), is a DNA-binding protein that is also referred to as a gene-regulatory protein or transcription factor. On the Y chromosome, SRY is a sex-determination gene without introns. A variety of sex development diseases caused by mutations in this gene have various phenotypic and genotypic impacts on the individual.

SRY is a DNA-binding protein that belongs to the SOX (SRY-like box) gene family. SRY functions as a transcription factor that upregulates other transcription factors, most significantly SOX9, when it interacts with the steroidogenic factor 1 (SF-1) protein. Primary sex cords form because of its expression and later transform into seminiferous tubules. The testis is formed when these cords develop in the centre of the undifferentiated gonad. The Sertoli cells subsequently begin to create anti-Müllerian hormone, while the testis' newly activated Leydig cells begin to secrete testosterone. The effects of the SRY gene often appear 6-8 weeks after foetus creation, and they prevent males from developing the feminine anatomical structures. Additionally, it works to improve men's secondary sexual traits.

Gene Evolution and Regulation

Evolution

The SOX3 gene, a member of the SOX family that is coupled to the X chromosome, may have undergone gene duplication to produce SRY. Following the separation between monotremes and therians, this duplication took place. Monotremes lack SRY, and a few of their sex chromosomes have similarities to those found in birds. SRY is a rapidly changing gene, and it has been challenging to investigate how it is regulated because animal sex determination is not a highly conserved process. The action of SRY varies between species, even among marsupials and placentals, which employ it in their sex determination process. The high-mobility group (HMG) box, which forms the centre of the gene, is conserved across species, although other parts of the gene do not. Only four genes on the human Y chromosome, including SRY, have been demonstrated to have originated from the original Y chromosome. The autosome that fused with the initial Y chromosome gave rise to the other genes on the human Y chromosome.

Regulation

Mice are the principal model research organisms that can be used for SRY studies since they share little in common with other model organisms' sex determination genes. Because there is limited protein sequence conservation even among mammalian species, understanding its control is even more difficult. The HMG box region, which oversees DNA binding, is the sole group that is conserved in mice and other animals. This area is the site of mutations that cause sex reversal, where the opposite sex is created. The SRY promoter, regulatory components, and control are not well understood since there is little conservation. There are similarities between related mammalian groups in the first 400-600 base pairs (bp) upstream of the translational start point. Human SRY promoter in vitro investigations have revealed that SRY promoter function necessitates a region at least 310 bp upstream of the translational start point. It has been demonstrated that binding of three transcription factors, including Wilms tumour protein 1, specificity protein 1, and steroidogenic factor 1, to the human promoter region affects the production of SRY. Two Sp1 binding sites, located at -150 and -13, in the promoter region serve as regulatory sites. A 90% reduction in gene transcription results from the mutation of the SRY binding sites in Sp1, a transcription factor that binds GC-rich consensus sequences. Studies on SF1 have shown less conclusive findings. Sex can be reversed because of SF1 mutations, and inadequate gonad development can result from deletion. However, it is unclear how specifically SF1 communicates with the SR1 promoter. There are two WT1 binding sites in the promoter region as well, located -78 and -87 bp from the ATG codon. WT1 is a transcription factor that mostly acts as an activator. It has four C-terminal zinc fingers and an N-terminal domain rich in proline and glutamine. Male gonad size is decreased by zinc finger mutations or WT1 inactivation. Complete sex reversal was the result of the gene's deletion. Although it is unclear how WT1 up-regulates SRY, some evidence indicates that it aids in stabilising message processing. This notion, however, is complicated by the fact that WT1 also regulates the expression of DAX1, which stands for dosage-sensitive sex reversal, adrenal hypoplasia crucial region, on chromosome X. In mice, DAX1 overexpression results in sex reversal. Although numerous alternative routes, such as RNA binding and SRY transcriptional destabilisation, have been proposed, it is still unclear how DAX1 works. Studies on the inhibition of male development have shown that DAX1 can interfere with SF1 function and, in turn, the transcription of SRY by enlisting corepressors.

There is also proof that GATA binding protein 4 (GATA4) and FOG2 cooperate with the promoter of SRY to activate it. FOG2 and GATA4 mutants exhibit much decreased amounts of SRY transcription, while it is unclear how these proteins regulate SRY transcription. There is no proof that FOG2 interacts with SRY, even though FOGs include zinc finger motifs that can bind DNA. According to studies, FOG2 and GATA4 connect with proteins that modify nucleosomes, which may activate them.

Functions

The cells of the primordial gonad that are located along the urogenital ridge are in a bipotential condition during gestation, which means they have the capacity to develop into either male or female cells (Sertoli and Leydig cells or follicular cells and theca cells), depending on the situation. By turning on male-specific transcription factors that enable these bipotential cells to differentiate and proliferate, SRY starts the process of testis differentiation. SRY achieves this via increasing the transcription factor SOX9, whose DNA-binding location is extremely like that of SRY. Fibroblast growth factor 9 (Fgf9) is upregulated by SOX9, which in turn causes SOX9 to be upregulated even more. The bipotential gonad cells start to develop into Sertoli cells once the appropriate SOX9 levels are reached. Additionally, the primordial testis will continue to form from cells that express SRY. The basic sequence of events is covered in this quick discussion, however there are a lot more variables that affect sex distinction.

1. Action in the nucleus

There are three primary areas in the SRY protein. The high-mobility group (HMG) domain, which functions as the DNA-binding domain and contains nuclear localization sequences, is in the middle region. The N-terminal domain can be phosphorylated to improve DNA binding; however, the C-terminal domain lacks a conserved structure. It starts with the acetylation of the nuclear localization signal regions of SRY, which enables importin and calmodulin to bind to SRY and facilitate its import into the nucleus. The testis-specific enhancer element of the Sox9 gene in Sertoli cell precursors, which is situated upstream of the Sox9 gene transcription start site, is found in the nucleus, and is bound by SRY and SF1 (steroidogenic factor 1, another transcriptional regulator). Particularly, the DNA target sequence's minor groove is bound by the HMG region of SRY, causing the DNA to flex and unwind. The creation of this specific DNA "architecture" makes it easier for the Sox9 gene to be transcribed. SOX9 specifically targets the Amh and prostaglandin D synthase (Ptgds) genes in the Sertoli cell nucleus. Amh can be produced by SOX9 binding to the enhancer nearby the Amh promoter, while prostaglandin D2 (PGD2) can be produced by SOX9 binding to the Ptgds gene. PGD2's autocrine or paracrine signalling promotes SOX9's entrance into the nucleus. After that, the SOX9 protein starts a positive feedback loop in which it functions as its own transcription factor, leading to the creation of significant amounts of SOX9.

2. SOX9 and testes differentiation

The SOX9 gene is little transcribed in the XX and XY bipotential gonadal cells along the urogenital ridge because of the SF-1 protein alone. However, only the XY gonad experiences significant up-regulation of the SOX9 gene because of the SRY-SF1 complex's binding to the testis-specific enhancer (TESCO) on SOX9. Transcription in the XX gonad is hardly detectable. Through a positive feedback loop, SOX9, like SRY, complexes with SF1 and binds to the TESCO enhancer, which promotes the expression of SOX9 in the XY gonad and contributes to some of this up-regulation. This up-regulation is also maintained by the proteins FGF9 (fibroblast growth factor 9) and PDG2 (prostaglandin D2). They have been demonstrated to be crucial for the continuation of SOX9 expression at the levels required for testis growth, even though their exact mechanisms are not entirely understood.

The cell-autonomous differentiation of supporting cell precursors into Sertoli cells, the first stage of testis formation, is thought to be mediated by SOX9 and SRY. The earliest Sertoli cells in the gonad's centre are thought to be the origin of a wave of FGF9 that travels throughout the growing XY gonad and promotes Sertoli cell differentiation by up-regulating SOX9. Although precise mechanisms are yet unknown, SOX9 and SRY are also thought to oversee a number of the later processes of testis development, including the differentiation of Leydig cells, the production of sex cords, and the development of testis-specific blood vessels. However, it has been demonstrated that SOX9 is essential for the development of the testes because it directly affects Amh, which encodes the anti-Müllerian hormone, when PDG2 is present.

SRY Disorders' Influence on Sex Expression

Embryos are gonadally similar up until the moment in development where the testis-determining factor induces the formation of male sex organs. The usual karyotype of a male is XY, while that of a female is XX. However, there are some exceptions where SRY is crucial. Klinefelter syndrome patients have numerous X chromosomes and multiple normal Y chromosomes, giving them the karyotype XXY. These people are categorised as men. When a sperm cell is maturing, atypical genetic recombination during crossover might produce karyotypes that do not correspond to their phenotypic expression.

The SRY gene often remains on the Y chromosome when a growing sperm cell goes through crossover during meiosis. The development of the testis will cease if the SRY gene is moved from the Y chromosome to the X chromosome. Swyer syndrome, with an XY karyotype and a feminine phenotypic, is what this is. The gonads are not functioning, yet the uterus and fallopian tubes are normally created in those who have this disease. People with Swyer syndrome are typically raised as females and identify as females. On the other end of the spectrum, XX male syndrome happens when SRY translocate to one of a body's female chromosomes. The XX male syndrome is characterised by a female karyotype but male physical characteristics. Both diseases can cause delayed puberty, infertility, and growth characteristics that correspond to the opposite sex from the one the person identifies with. Swyer syndrome sufferers may get facial hair, while XX male syndrome expressers might have breasts.

It has been proposed that there are other elements that affect SRY's functionality, even though SRY has typically been associated with whether testis development happens. As a result, some people with the SRY gene nonetheless experience female development, either because the gene is damaged or mutated, or because one of the contributing components is. Individuals with an XY, XXY, or XX SRY-positive karyotype may experience this.

In addition, activities that occur after SRY is present or absent throughout an embryo's development are other sex-determining systems that depend on SRY in addition to XY. If SRY is present for XY in a typical system, SRY will activate the medulla to cause the development of gonads into testes. The subsequent production of testosterone will start the process of developing additional male sexual features. Comparatively, since XX lacks a Y chromosome, SRY will also be absent if it is absent for XX. Lack of SRY will allow the ovaries to develop from the cortex of embryonic gonads, which will cause the ovaries to release oestrogen and cause the development of other female sexual traits.

Role in Other Diseases

Due to an underlying androgen insensitivity syndrome (AIS), people with an XY karyotype and a functional SRY gene may have an overtly female phenotype. SRY has been demonstrated to interact with the androgen receptor. A deficiency in the androgen receptor gene causes people with AIS to be unable to respond to androgens appropriately. Affected people may have total or partial AIS. Males are more likely than girls to suffer dopamine-related disorders including schizophrenia and Parkinson's disease, and SRY has been connected to this fact. Dopamine, a neurotransmitter that transmits messages from the brain that regulate movement and coordination, is controlled by a protein that is encoded by SRY. An SRY-encoded transcription factor called SOX10 has been connected to the mouse disease known as dominant megacolon, according to research in mice. The relationship between SRY and Hirschsprung disease, or congenital megacolon in humans, is being studied using this mouse model. Additionally, there is a connection between CD and the transcription factor SOX9 that is expressed by the SRY gene. Skeletal CD is a result of a missense mutation that impairs chondrogenesis, or the process of cartilage development. Approximately two thirds of 46, XY people with CD exhibit varying degrees of male-to-female sex reversal.

Use in Olympic screening

One of the more contentious applications of this discovery included a system put in place by the International Olympic Committee in 1992 for gender verification during the Olympic Games. Although all athletes in whom this was "detected" at the 1996 Summer Olympics were considered false positives and were not disqualified, athletes with an SRY gene were not allowed to compete as females. In particular, the SRY gene was discovered in eight female competitors (out of a total of 3387) in these games. However, further genetic examination revealed that all these athletes were female, and they were all permitted to compete. Despite having the SRY gene, these athletes were discovered to have either partial or full androgen insensitivity, making them phenotypically female. In the United States, a number of pertinent professional bodies, notably the American Medical Association, campaigned for the removal of gender verification in the late 1990s, claiming that the technique was unreliable and ineffective. Chromosomal screening was discontinued as of the Summer Olympics in 2000, however other types of testing based on hormone levels took its place.

Ongoing Research

Despite the advancements in the understanding of sex determination, the SRY gene, and its protein over the past few decades, research is still being done in these fields. The chromosomal abnormalities implicated in many other human sex-reversal cases are still unknown, and there are still components in the sex-determining molecular network that need to be uncovered. Researchers are still looking for additional sex-determining genes using methods like mutagenesis screens in mice for sex-reversal phenotypes, microarray screening of the genital ridge genes at different developmental stages, and chromatin immunoprecipitation to identify the genes that transcription factors act on.